Purposeful Coexistence: Managing Dual-Run States in Core Modernization

Modernizing a bank’s core systems is no longer a question of “if,” but “how.” As digital-first challengers and rising customer expectations reshape the financial landscape, banks must move beyond legacy technology to deliver seamless, personalized, and agile experiences. Yet, the journey to a modern core is fraught with complexity and risk—none more daunting than the challenge of managing dual-run, or coexistence, states where legacy and new core systems operate in parallel. This page provides a deep dive into the coexistence migration pattern, outlining why big bang migrations are increasingly rare, the benefits and complexities of coexistence, and best practices for planning, executing, and decommissioning legacy systems.

Why Big Bang Migrations Are the Exception, Not the Rule

Historically, banks often attempted “big bang” migrations—moving all products and data from legacy to new core systems in a single, high-stakes event. While this approach was feasible in an era of limited digital channels and predictable downtime windows, it is now widely recognized as risky and impractical. The batch-based nature of legacy cores, the need for 24/7 digital banking, and the regulatory scrutiny following high-profile failures have made big bang migrations the exception. Today, most banks accept that a phased, coexistence approach is the safer, more effective route to modernization.

The Coexistence Pattern: What It Is and Why It Matters

Coexistence refers to running legacy and new core systems in parallel for a defined period. This pattern allows banks to migrate products, customers, and processes in controlled tranches, reducing risk and enabling continuous learning. Coexistence aligns with modern software practices, supporting iterative deployment, testing, and refinement. It also enables banks to:

De-risk migration events by limiting the blast radius of any issues
Maintain business continuity and customer experience throughout the transition
Align migration with regulatory and operational requirements
Enable creative deployment strategies that can adapt to real-world challenges

The Complexities of Coexistence

While coexistence offers significant risk mitigation, it introduces its own set of challenges:

Increased operational complexity: Running two (or more) cores in parallel requires robust processes, clear ownership, and strong governance.
Data consistency and reconciliation: Ensuring data integrity across systems is critical, especially as products and customers move between cores.
Integration and routing: Banks must manage how channels, payments, and downstream systems interact with both legacy and new cores, often requiring a sophisticated routing layer.
Change management: Coexistence impacts teams across the organization, demanding clear communication, training, and alignment.
Cost management: Prolonged dual-run states can increase costs if not carefully planned and executed, especially if legacy decommissioning is delayed.

Best Practices for Managing Coexistence

Drawing on extensive experience delivering core modernization programs, Publicis Sapient recommends the following best practices for successful coexistence:

1. Embrace Coexistence as a First-Class Concept

Treat coexistence as a critical program phase, not a footnote. Empower a central team with the authority and expertise to plan, coordinate, and make decisions across business and technology domains. This team should be distinct from those focused solely on the target state.

2. Plan for Multiple Transition States

Recognize that coexistence is not a single state, but a series of transition states (e.g., T0: legacy only, T1: dual-run, T2: new core only). Define clear criteria for entering and exiting each state, and use these as milestones for program governance.

3. Leverage Technology Enablers

Routing Layer: Implement a routing layer to direct transactions and data flows to the appropriate core. This layer should be flexible, allowing for efficient and transparent routing decisions as migration progresses.
Common Data Layer: Build a data aggregation platform that feeds downstream systems (e.g., general ledger, reporting) from both cores, minimizing downstream impact and simplifying reconciliation.
Automated Reconciliation: Invest in automated, repeatable reconciliation capabilities to ensure data quality and quickly identify and resolve discrepancies.

4. Tactically Invest in Legacy Systems

Make targeted changes to legacy systems to support coexistence (e.g., adding migration flags, enabling new data extracts). Engage legacy teams early and plan for required changes well in advance.

5. Separate Migration and Coexistence Functions

Migration (moving data from A to B) and coexistence (operating in dual-run states) are distinct. Assign clear ownership and objectives to each, ensuring that migration teams are not overburdened with coexistence responsibilities.

6. Go Deep on Data Analysis

Profile and understand your legacy data in detail. Use this insight to define migration tranches (by customer, product, or other attributes) that minimize complexity and customer impact.

7. Test and Prove in Production Early

Where possible, pilot coexistence with a small set of customers or products in the live environment. Use these early tests to validate end-to-end behavior and de-risk larger migration waves.

8. Plan for Decommissioning from Day One

Define the criteria and timeline for decommissioning legacy systems early in the program. Track progress against these milestones to ensure cost savings and realize the full benefits of modernization.

Organizational Strategies for Success

Centralized Coexistence Team: Empower a cross-functional team with senior sponsorship to manage coexistence, make rapid decisions, and align stakeholders.
Change Management: Communicate regularly and transparently with all impacted teams. Provide training and support to ensure smooth adoption of new processes and systems.
Continuous Learning: Use retrospectives and feedback loops to refine migration strategies, address issues, and accelerate future waves.
Stakeholder Alignment: Secure buy-in from business, technology, operations, and risk teams. Coexistence impacts the entire organization and requires broad commitment.

Lessons Learned from Real-World Programs

Don’t try to answer every question up front: Coexistence is complex and context-specific. Set a clear direction, but be prepared to iterate and solve challenges incrementally.
Invest in technology, not manual workarounds: Modern architectures and automation reduce operational risk and enable faster, safer migrations.
Early and ongoing data analysis is critical: Understanding your data landscape enables smarter migration planning and reduces surprises.
Decommissioning is a program in itself: Without a clear plan and accountability, legacy systems can linger, eroding the business case for modernization.

The Future: Coexistence as the New Normal

As banks continue to digitize their core, coexistence is here to stay—especially for large, complex institutions. Big bang migrations will be reserved for smaller markets or specific products. Treating coexistence as a first-class concept, with dedicated teams, technology enablers, and robust governance, is essential for a safe, phased transition to a modern core.

How Publicis Sapient Can Help

Publicis Sapient brings deep expertise in core modernization, migration, and coexistence strategies. Our proven methodologies, technology accelerators, and cross-functional teams help banks de-risk their journeys, accelerate value realization, and deliver the digital experiences customers expect. Whether you are just starting your modernization journey or navigating the complexities of coexistence, we are ready to help you succeed.

Ready to modernize your core? Let’s build the future of banking—together.

For more information, visit publicissapient.com or contact our core modernization experts.